Both the meridional muscle bundles and the circular muscle bundles of the acetabulum will, upon contraction, decrease the circumference and thereby increase the thickness of the acetabular wall.

These muscles originate on a connective tissue layer that surrounds the arm musculature and extend down to converge on the sucker and insert on the outer connective tissue capsule of the acetabulum at the level of the sphincter muscle.

The muscle volume has to go somewhere, so the circumference of the cup-shaped acetabulum has to increase, increasing the volume of the acetabular chamber.